During a special meeting Friday, about 90 percent of shareholders approved the merger agreement despite recent pushback from activist shareholder Carl Icahn and Sen. Chuck Grassley from Iowa, the top Republican on the Senate Judiciary Committee. The lopsided approval was expected, according to industry experts.

The U.S. Department of Justice is performing an antitrust review of the proposed merger, which is expected to be completed later this year. Cigna said it expects the deal, subject to customary closing conditions and regulatory approvals, to close by year-end 2018.

In July 2016, the Justice Department sued to block the $37 billion Aetna-Humana merger and another acquisition planned between Anthem and Cigna.

Cigna CEO and President David M. Cordani said the insurer is "delighted" that shareholders have pushed along the deal projected to improve affordability and healthcare choice for consumers and add value for investors.

"Our combined company will enhance Cigna's differentiated service-based model, fueled by actionable insights and analytics, to drive innovation and meaningful growth in a highly dynamic market environment," Cordani said in a statement Friday.

In March, Cigna agreed to buy Express Scripts, one of the nation's biggest pharmacy benefits managers that administers drug plans for patients with employer and government health insurance.

The deal signals a major consolidation effort in the healthcare industry, which also includes CVS Health's proposed deal to acquire Hartford health insurer Aetna for $69 billion.